The present invention relates to an implantable stimulating lead for a cardiac pacemaker.
In physiological terms, a cardiac pacemaker must be capable of generating a signal with a sufficient magnitude to depolarise the excitable cells of tissue within the heart. This signal is delivered to the cardiac tissue of the the heart via a lead which has an electrode tip in contact with the heart tissue. Electrode size and shape, tissue electrolyte conductivity, and the distance separating the electrode from the adjacent tissue are factors in determining the energy required of the pacemaker. Many of these factors are affected by the particular geometry and material composition of the electrode, as explained hereinbelow.
For example, current drain in a constant voltage pacemaker is determined by a combination of the impedance of the pacemaker circuitry, the nature of the electrode resistance and the characteristics of the electrode tip interface with the surrounding tissue. The most significant frequency component of the pacing pulse generated by the pacemaker is on the order of 1 KHz. At this frequency, most of the impedance to the pacing pulses is due to the bulk of the electrode, i.e. "spreading" impedance.
The impedance presented to the pulse generated by the pacemaker is a function of the geometric, i.e. macroscopic, surface area of the electrode and the radius of the electrode. For example, an electrode having a small radius will have a higher pacing impedance and smaller current drain than a similarly shaped electrode of a larger radius. All of these factors must be considered in maximizing the design of an electrode for purposes of effectively delivering pulses so as to pace operation of the heart.
In addition to this pacing function, the electrode must also provide for sensing of heart activity, e.g. for determining the presence of aberrant behavior so that pacing operation will be initiated. In this sensing operation, the most significant sensed frequency components of atrial or ventricular signals are in the bandwidth of 20-100 Hz. In this region, interface impedance of the electrode with the surrounding cardiac tissue becomes significant. This impedance is determined by the microscopic surface area of the electrode and is established within a few microns of the electrode's surface. The microscopic surface area, or microstructure of an electrode, is the total surface area, including all microscopic ridges, cracks, crevices and indentations on the stimulating surface of the electrode.
Another factor of concern in connection with maximizing operation of pacemaker electrodes relates to pacing threshold. The pacing, or stimulating, threshold is a reflection of the energy required for a pulse to initiate a contraction in the cardiac tissue. This stimulation threshold rises for weeks after the implant of a pacemaker lead as a result of an increase in the spacing between the electrode and the excitable tissue. The spacing occurs due to the development of a fibrous capsule around the electrode tip which is reported to be between 0.3 mm and 3 mm thick. There are indications that lower long term pacing thresholds result with more reliable fixation of the electrode to the surrounding tissue.
In view of the above characteristics of an electrode for a cardiac pacemaker, an electrode should have a small geometric macroscopic surface area and a small radius in order to provide high pacing impedance and low current drain. However, to achieve low sensing impedance and thus enhance sensing, the same electrode tip should have a large microsurface area or enhanced microstructure. Furthermore, to provide lower long term pacing thresholds, the electrode should also provide secure and reliable attachment to the heart wall with minimal fibrous capsule formation.
Heretofore, in order to achieve the foregoing, pacemaker leads were provided with an electrode that is both porous and conductive. In devices of this type, the conductive characteristics were adapted to provide the electrical functions, i.e. sensing and pacing operations, while the porous characteristics were relied upon to facilitate attachment to the cardiac tissue by promoting tissue ingrowth. Such devices suffer in design, however, in that the single surface area of the electrode must satisfy the various, and oftimes contradictory, design demands as outlined above.
Specifically, although electrodes can be designed that are satisfactory for the purposes of pacing and sensing, the desire for a porous conductive tip structure is often difficult to reconcile. Furthermore, such small geometric surface areas are difficult to construct, and it is necessary to limit any reduction in electrode diameter in order to minimize the risk of cardiac wall perforation. An additional problem results in that stimulation electrodes are generally made of expensive metals (pt/Iridium, Pt), so any additional conductive material required in making the external stimulation surface and tissue ingrowth structure into one unit significantly increases the cost of manufacture.